Doppler effect of a wave is defined as the change in frequency of the wave from an observer side that is moving relative to the source of the wave. For Doppler effect, the frequency of the received (reflected) wave is higher during the approach, identical at the instant of passing by, and lower as the source move away from the observer (receiver), relative to the frequency of the emitted wave. In other words, each successive wave peak is emitted from a position closer to the observer than the previous wave, when the source of the waves is moving toward the observer. As a result, each wave takes slightly less time to reach the observer than the previous wave and thus the time between the arrival of successive wave peaks at the observer is reduced, causing an increase in the frequency. While the waves are travelling, the distance between successive wave fronts is reduced causing the waves to bunch together. On the other hand, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency, when the source of waves is moving away from the observer. The distance between successive wave fronts is increased, so the waves spread out.
Doppler effect is utilized in a variety of different application, for example, measuring fluid flows, temperature, vibration and velocity, by a Lidar or laser.
An inelastic scattering of light (photons) is when photons are scattered from an atom or molecule, causing most photons to elastically scatter (Rayleigh scattering), such that the scattered photons have the same frequency and wavelength as the incident photons. A dynamic or quasi-elastic scattering of light (photons) is a scheme that can be used to determine the size distribution profile of small particles in suspension or polymers in solutions and to examine the behavior of complex fluids such as concentrated polymer solutions.
When light hits small particles, the light scatters in all directions (Rayleigh scattering) as long as the particles are small compared to the wavelength of the light. If the light source is a laser (monochromatic and coherent), then a time-dependent fluctuation in the scattering intensity can be observed. This fluctuation is due to the fact that the small molecules in solutions are undergoing Brownian motion, and so the distance between the scatterers in the solution is constantly changing with time. This scattered light then undergoes constructive or destructive interference by the surrounding particles, and within this intensity fluctuation, information is contained about the time scale of movement of the scatterers.
For measuring the wind velocity, atmospheric aerosol and molecular back-scattering is used. However, conventional edge detection and spectral binning wind lidar systems use low finesse etalons to measure Doppler frequency shifts and extract the wind velocity. However, existing approaches are too slow and have too low a velocity precision to meet some higher speed and accuracy of some applications, because they are also photon-inefficient.
Doppler shifted off-resonance light scattering from fluids (condensed phase and gas) is important to characterizing flow dynamics in many applications. However, for fluid flows without particles in them (hydrosols, aerosols) conventional quasi-elastic light scattering measurements have low signal to noise ratio (SNR).